Digital photography is one of the most exciting technologies that emerged in the past ten years. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Digital still and video cameras are on the cutting edge of digital photography. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make the digital cameras the hottest new category of consumer electronics products.
Unlike traditional cameras that use film to store an image, digital cameras use a photosensitive device called an image sensor. Until recently, the dominant and nearly only solid-state image sensor technology was the charge-coupled device (CCD). However, most microprocessors, logic circuits, ASICs (application-specific integrated circuits), and memory circuits are based on Complementary Metal Oxide Semiconductor (CMOS) technology. Logically, CMOS will be used for image sensors in a rapidly expanding sphere of applications.
Several important factors have contributed to the emergence of CMOS image sensor arrays at this time rather than 10 years ago. The primary factor is a recent demand by customers for portable, low power, miniaturized digital imaging systems. A second important factor is that current CMOS technology offers submicron feature sizes and low defect and contamination levels, respectively permitting cost-effective pixel sizes and low junction leakage (or dark) current. In addition, threshold voltage control and uniformity is stable and reproducible. The third important factor is that new circuit techniques have been invented or have been adopted from CCD signal processing that permit both low noise and high-dynamic-range imaging that is competitive with the best CCDs.
A CMOS image sensor array comprises millions of photocells or photo sensors. Each of the photo sensors produces an electronic signal representing the intensity of the light that reflects from a target and falls on it by accumulating a charge; generally the more light, the higher the charge is. After the light integration, the electronic signals from all of the photo sensors are readout and then digitized through an analog-to-digital converter to produce digital image of the target.
FIG. 1 illustrates an exemplary use of a CMOS image sensor in an imaging sensing system 100 such as a digital camera. The image sensing system 100 comprises an optical system 132 for focusing an image onto image sensor array device 130. Readout circuitry 133 reads an electronic image information from the image sensor array 130. The readout circuitry 133 passes analog image signal representing the target image is to an analog-to-digital converter (A/D) 140 for digitization.
The A/D converter 140 digitizes the analog image signal from the image sensor 130 to create a digitized signal that can be appropriately and subsequently stored in memory 150. Typically, an imaging system 100 further comprises a digital signal processing circuitry 160 that performs various processing tasks to put the digital image into a suitable form. For example, the digital signal processing circuitry 160 may adjust, correct, preprocess and compress the digitized signal to eventually output an appropriate digital image or signal. In the embodiment of FIG. 1, the digital signal processing circuitry 160 performs operations necessary to place the image signal into suitable form for display onto display 171 using display driver 170.
As shown in FIG. 1, traditional image sensor arrays could only output image signals that required further signal conditioning before such image signals could be used. For example, the imaging system 100 required digital signal processing circuitry 160 to processes the image signal before it could be passed to the display system. To simplify the design of an image system, it would be desirable to eliminate or reduce the need for a digital signal processor in an imaging system.